Introduce CKEditor 5 builds

[pomek]

In order to create the CKEditor 5 builds, we need to do some changes in existing packages:

• create a new package - ckeditor5-dev-bundler-webpack which will produce the bundle using Webpack,
• ckeditor5-dev-utils - create a function which will create an entry file (for both bundlers),
• ckeditor5-dev-bundler-rollup - update the package, it should use the function (for creating the entry file) added in utils package.

[pomek]

Because package ckeditor5-dev-bundler-webpack does not make a sense, we will merge utils from this package into ckeditor5-dev-webpack-plugin and rename as ckeditor5-dev-webpack-utils.

[Reinmar]

At the end, we only needed to expose more utils in ckeditor5-dev-utils. The rest of the code landed in the build repo.